CGT only applies from 31 March 1982 so the value of the flat at that point should be used but as you bought it less than a year before, I have used £21,000 as part of my calculations.
For CGT purposes, the flat cost you £147,000 (£21,000 / 2 + £8,500 - £9,500 + £137,500). If you sell it in, say, December 2016, for £750,000 you will make a gain before the deduction of the expenses of purchase and sale of £603,000 (£750,000 - £147,000). By that time, you will have owned it for 417 months from 1 April 1982 of which you will have lived in it for 131, let it for 280 and it will have been vacant for 6.
The gain for the period the property was your main home will be exempt from CGT as will the gain for the last 18 months of ownership. That accounts for £215,461 (£603,000 / 417 months x 149 months). The remaining gain of £387,539 is split as to £378,863 to that part of the letting period gain which is not covered by the last 18 months of ownership and £8,676 to that part of the vacant period gain not covered by the last 18 months of ownership.
As the property was both your main home and it was let you will be entitled to letting relief which will be the lesser of:
2 the sum of the main residence period gain and the gain for the last 18 months of ownership of the property which is £215,461 and
3 the letting period gain of £378,863.
Letting relief of £40,000 will reduce the remaining gain of £387,539 to £347,539 and the annual CGT exemption of £11,000 (the current rate) will reduce it further to leave you with a net taxable gain of £336,539.
There are two rates of CGT, 18% and 28%. The rate or combination of rates that you will pay will be dependent on the level of the sum of your income and the taxable gain in the tax year of disposal of the flat. Using the tax bands for the current year (we don't know what they will be in 2016/17) and assuming the same CGT rules don't change by 2016/17, one of the following scenarios will apply:
1 If your income in 2016/17 including the taxable gain is £41,865 or less, then all the taxable gain will be taxed at 18%.
2 If your income in 2016/17 excluding the taxable gain is more than £41,865, then all the taxable gain will be taxed at 28%.
3 If your income in 2016/17 excluding the taxable gain is less than £41,865 but more than £41,865 when you include the taxable gain then part of it will be taxed at 18% and part at 28%.
I'm afraid that short of moving into the flat and living there for a long time, there really isn't much you can do to mitigate the potential CGT. If you sold your current home and bought another property to live in and stopped letting the flat, you could elect for the flat to be your main home but that election would only apply from the date you make it.
If you are married, you could put the flat in joint names but you and your spouse would both have to be living there (it has to be only or main home) when you did that for the main residence relief and letting relief you have already accrued to be split between the two of you. If you were not and you put the property in joint names, on a future disposal, your spouse lose out on the already accrued main residence relief and letting relief. Many people fall foul of that rule. Read about it here. The point of the property being in joint names would be to have more of the taxable gain charged at 18% as opposed to 28% but whether that would be the case would be dependent on the level of your spouse's income.
If you don't sell the flat until you retire, you may benefit from paying less CGT at 28% if you are a basic rate taxpayer at that time.
If you moved abroad, you may only be able to reduce your exposure to CGT in the UK but you may pay it in the country you establish tax residence in. I won't go into the rules as they are changing in April 2015 so that non-UK resident owners of UK residential property will become liable to UK CGT on their gains from selling such property but only on the gain accrued after 5 April 2015. The detailed rules have yet to be decided so, if that is something that may interest you, look out for the 2015 Budget announcements.
I hope this helps but let me know if you have any further questions.